Units of this type are used in the nuclear industry for cleaning the water to be taken into the reactor, more precisely by deionisation, primarily in respect of Fe ions but also by removal of undesirable pollutants. To this end, the slender treatment tubes are provided with perforations and surrounded by a cloth whose outside is coated with an ion-exchange medium. In actual practice, the treatment tubes are so mounted in an upper chamber as to be separate from an open, lower chamber, the treatment tubes being closed at the bottom and open at the top. Thus, water entering the tubes from outside will first penetrate the external layer of ion-exchange medium and then pass through the small perforations in the circumferential walls of the tubes, before discharging into the upper part of the container, where it is collected in a throughgoing outlet tube extending centrally through the lower, free chamber and out through the bottom of the container.
In prior-art units of this type, the inlet consists of a tube which opens directly into the lower chamber of the container, more precisely in the cylindrical wall, with the tube projecting radially therefrom. One problem which is difficult to master in these known units is that the flow velocity of the water becomes extremely uneven outside the separate treatment tubes owing to uncontrolled turbulence in the lower chamber. Thus, the velocity of the water flowing past the treatment tubes can be extremely high at some tubes while being comparatively low at others. Where the water passes at a high speed, the ion-exchange medium on the tubes is quickly eroded, which shortens the time of operation elapsing between the required restorations of the medium. These restorations being troublesome and time-consuming operations, the too short times of operation therebetween will detract from cost-effectiveness.